1. Field of the Invention
The present invention relates to techniques for inspecting, qualifying and repairing a photo-mask for use in photolithography at extreme ultra-violet wavelengths.
2. Related Art
Photolithography is a widely used technology for producing integrated circuits. In this technique, a light source illuminates a photo-mask. The resulting spatially varying light pattern is projected onto a photoresist layer on a semiconductor wafer by an optical system (which is referred to as an ‘exposure tool’). By developing the 3-dimensional (3D) pattern produced in this photoresist layer (which is sometimes referred to as a ‘target pattern’), a layer in the integrated circuit is created. Furthermore, because there are often multiple layers in a typical integrated circuit, these operations may be repeated using several photo-masks to produce a product wafer.
In order to appropriately scale to smaller critical dimensions in integrated circuits (and, thus, to reduce diffraction and proximity effects that occur when light is propagated through the optics of the exposure tool and is converted into the 3D pattern in the photoresist), commensurately smaller wavelengths of light may be provided by the light source. However, it is difficult to design and manufacture transmission photo-masks at small wavelengths, such as in the extreme ultra-violet (EUV).
Recently, reflective or EUV photo-masks have been investigated for use with wavelengths in the extreme ultra-violet. In an EUV photo-mask, a multilayer stack is used to reflect the light from the light source. For example, multiple alternating layers of silicon and molybdenum may be deposited on silicon or a glass substrate having an ultra-low thermal expansion coefficient (such as quartz). Then, the mask pattern may be defined in an absorption layer (such as tantalum nitride) that is deposited on top of the multilayer stack.
In practical reflecting photo-masks, up to 80 alternating layers are used. Furthermore, these layers may have thicknesses as small as 2-4 nm. However, this structure can be difficult to manufacture. For example, during the manufacturing process defects can occur throughout the multilayer stack. It can be difficult to detect the presence of a defect in the multilayer stack without performing destructive analysis. In addition, even if a defect is detected (or when a type of defect is probable in a given manufacturing process), it is often difficult (or impossible) to predict the consequences of the defect in the photolithographic process (e.g., will the defect result in an unacceptable change in the 3D pattern) or to determine how to modify an EUV photo-mask to reduce or eliminate the effect of a defect on the photolithographic process. As a consequence, the inspection and qualification criteria for EUV photo-masks are often needlessly conservative, which results in rejection of EUV photo-masks that could be successfully used in the photolithographic process (i.e., the EUV photo-mask yield may be needlessly reduced), thereby significantly increasing the cost of EUV photo-masks. In addition, it is often difficult to appropriately modify the photo-mask to reduce or eliminate the effect of the defect on the photolithographic process, which also adversely impacts the EUV photo-mask yield, and this also increases the cost of EUV photo-masks.
Hence, what is needed is an EUV photo-mask inspection and qualification technique that overcomes the problems listed above.